The present invention relates to a flotation mechanism comprising a directional element and vertical vanes located in a flotation cell. The directional element is symmetrical and is fixed at the centre to the lower section of the hollow shaft of the mechanism. According to the corresponding method, due to the directional element, which is cylindrically inclined outwards from the outer edge the flotation mechanism directs the gas-slurry suspension that is formed in a downward slanting direction towards the side wall of the cell. The mineral suspension rises upward from the sidewall towards the centre of the cell, from where the flow is diverted to the edges of the cell and the froth generated is removed from the cell. Using this flotation mechanism enables a powerful agitation, extending throughout the entire mixing zone of the flotation cell.
Flotation cells may be single mixing vessels, in series or in parallel. They may be either rectangular or cylindrical in shape, in horizontal or upright position. Gas is routed through the hollow mixing shaft to the small rotating rotor on the bottom. The rotor causes a powerful suction as it rotates, which sucks the gas into the rotor space. In the rotor space the slurry is mixed with the gas bubbles discharging and dispersing via the shaft. Usually a stator built of vertical plates is installed around the rotor, which promotes gas dispersion and attenuates the rotation of the slurry. Mineral particles stuck to the gas bubbles rise from the stator to the surface of the froth layer and from there out of the cell into the froth launders.
Nowadays it is becoming increasingly common to use upright cells, which are also cylindrical and normally flat-bottomed. One problem with flotation cells is sanding, i.e. solid matter builds up on the bottom of the cell in an immovable layer. This is usually due to a too small or ineffective rotor, as in such a case the mixing zone of the rotor does not extend far enough. Another common difficulty is that the mineral particles already attached to the gas bubbles cannot be removed from the flotation cell, because the flows forming in the cell and particularly at its surface and upper section are wrongly oriented or too weak i.e. they are not able to move the flotated gas bubbles out of the cell.
A flotation mechanism is known in the prior art according to U.S. Pat. No. 4,078,026, where the gas to be dispersed is conveyed via a hollow shaft to the inside of a rotor rotating on said shaft. The rotor is designed in such a way as to preserve a balance between the hydrostatic and dynamic pressure, that is, the vertical section of the rotor is a downward narrowing tapered cone. The rotor has separate slurry slots for slurry and gas.
The so called Svedala mechanism known before by EP patent 844 911 deals with a mixer fixed to an upright shaft for mixing gas and slurry. In this mixer there are several vertical plates radially around the shaft and between the plates there is a horizontal baffle around the shaft, with a width of about half that of each plate. Gas enters below the baffle. The parts of the mixer above the baffle cause first a downward flow, which then at the baffle becomes an outward flow and correspondingly the parts below the baffle cause first an upward and then outward flow, as shown in FIG. 3 of the patent. The outer edges of the blades of the mixer are straight at their upper part, but the lower parts narrow inwards in a concave fashion. There is a stator around the mixer.
U.S. Pat. No. 5,240,327 describes a method of mixing different phases particularly in a conditioning cell. In connection with the method, there is described the zones creating in the reactor and a controlled flow dynamic in order to achieve zone distribution. The patent describes a cylindrical, flat-bottomed upright reactor, wherein are vertical baffles in order to attenuate the turbulence of the slurry. In addition the reactor has a ring-shaped horizontal baffle (back-flow guiding member) in order to guide the vertical flows and divide the reaction space in two. The patent further describes a special mixer with which to obtain the desired flow dynamics. This arrangement thus enables the formation of a double toroid in the section below the horizontal guiding member thanks to the combined effect of the horizontal guiding member and the mixer, wherein the slurry fed into the lower section first swirls in the lower bottom toroid and then gradually shifts to the upper toroid. From here the well-mixed dispersion rises into the pacified and controlled flow zone situated above the guiding member and is then removed via an overflow aperture. The double zone model described in the patent is suitable for normal chemical reactions and particularly for the flotation and conditioning of mineral concentrates.
A mineral slurry conditioning cell is known from U.S. Pat. No. 5,219,467, which is in some way a further development of the method and equipment mentioned in the previous patent. The apparatus comprises a colon-like reactor, in which concentration takes place in three separate zones. The reactor is equipped with upright flow guides, a horizontal flow attenuator and a mixer. Flotation reactions are created in the bottom zone, from where gas bubbles and mineral particles carried by them are directed to the surface of the apparatus. The apparatus is designed so that a strong agitation can be used in the bottom zone without harming the separation of the froth in the upper zone.
Now a new flotation mechanism has been developed, which achieves a powerful agitation in the coverage area of the mixer, which extends throughout the whole lower zone or mixing zone. The mechanism or mixer disperses the flotation gas into fine xe2x80x9cmilkyxe2x80x9d bubbles. It is advantageous to feed the gas via the shaft of the mixer. The mixer sucks the slurry both up and down and mixes it effectively into the bubbles of gas being generated.
Thanks to its guiding element, which is cylindrically inclined from the outer edge, the mixer directs the gas-slurry-solid suspension formed at a downward angle towards the inner wall of the cell. The flotation mechanism according to this invention fulfils for instance the requirements presented for the latter described mechanism. Furthermore, the mixer is not only effective but also its structure is balanced, strong and above all simple.
The flotation mechanism of this invention can be called glsdl (gas-liquid-solid-dispersion-lap). The purpose of the apparatus according to this invention is to disperse the flotation gas into small, fine bubbles that are evenly distributed in the slurry, to develop a strong turbulence in the immediate range of the mixer i.e. agitation intensity, and to prevent in this way coarse particles from settling on the bottom of the flotation cell. Another purpose is to create the kind of flow in the flotation cell described in the previously-mentioned patent, in other words, to generate a toroidal flow in the mixing zone directed down from the mixer to the side walls, and correspondingly above the mixer a toroidal flow directed upwards from the mixer to the side walls. The agitation intensity is several kilowatts per cubic meter of slurry. Using the cell constructions of the prior art (vertical and horizontal guiding elements), a part of the toroidal flow is routed via the pacified zone to the upper zone, from where the mineral particles with the gas bubbles rise to the froth layer, and from there to the froth launders around the cells.
According to this invention the flotation mechanism consists of two parts: the directional element and the upright vanes. The directional element is symmetrical and is fixed at the centre to the lower section of the hollow shaft of the mechanism. The central section of the directional element, i.e. the part directed outward from the shaft is a horizontal circular plate, which is folded downwards at its outer edge in the shape of a truncated cone. The downward folded outer edge forms an angle xcex1 with the horizontal plane, preferably between 30-60xc2x0, and this directional element lap forms the actual guiding element. Vertical vanes are fixed to the directional element, numbering at least four, but preferably six. These vertical vanes extend above and below the directional element and sideways preferably right up to the outermost edge of the directional element. The width of the vertical vanes is advantageously greater than that of the conical lap of the directional element and thus the inner edge of the vanes extends as far as the horizontal plate. It is also preferable to place a horizontal guiding plate on the inside of the directional element, to direct the gas discharging via the shaft to the side towards the directional element lap. The essential features of the invention will become apparent in the attached claims.
The outer edge of the flotation mechanism vanes is substantially vertical, whereby the most effective dispersion of flotation gas is achieved, i.e. the maximum underpressure is generated behind the vane. The inner edge of the vane is vertical at the top, but narrowing in a curve at the bottom designed this way with the purpose of minimising energy loss. The curve preferably follows the shape of a circular arc, where the centre point of the circle is the outer edge of the vane. The advantage of a downwardly narrowing vane is also the fact that the mechanism is easy to restart after a stop, regardless of the settling slurry around it.
The mixing/flotation mechanism of this invention works even without stators, but as has been found in flotation, this mechanism also functions more effectively when using stators around it. The stator is in such a case a conventional one i.e. it comprises upright, rectangular-shaped plates. The stator attenuates the turbulence and also the flow of the slurry to some degree, but nevertheless it does not xe2x80x9cspoilxe2x80x9d the basic idea of the mechanism. The positive impact of the stator is that it balances out the distribution of energy in the mixing zone.